In communication loops utilizing telephone lines, a subscriber's device, e.g. a telephone, is normally connected across the telephone lines. When the subscriber's device is activated, a current is drawn, which is termed the loop current. A central station recognizes the increase in loop current as an indication that the subscriber's device is operational. For a typical telephone line, the current available from the central station is in the range of 20 to 100 milliamps. This is due to the telephone company using lines of different impedances and lengths. The current drawn through the telephone lines results in a voltage drop across the line. The voltage level at the central station is normally 48 volts whereas the voltage at the subscriber's device is in the range of 4 to 10 volts. However, in certain applications, the telephone company utilizes a subscriber carrier system to extend a telephone line to an additional subscriber. The subscriber carrier system provides a lower voltage to the added subscriber and has only little current drive capability. Since there is no way of knowing in advance the application in which the subscriber device is to be used, the subscriber's device must be operable with any length of conventional telephone line or on a subscriber carrier system.
Due to the use of relatively high impedance telephone loop systems, there exists a need for a circuit that senses the lower loop current available and automatically compensates the subscriber's device to work with the lower loop current system. This alleviates any need for a special subscriber device to operate with a high impedance loop.